1. Field of the Invention
The invention relates to a processing head for a laser processing device, with which workpieces can be welded, cut or otherwise processed.
2. Description of the Prior Art
Laser processing devices normally comprise a laser radiation source which may, for example, be a fibre laser or a disc laser. A laser processing device further includes a processing head, which focuses the laser radiation generated by the laser radiation source in a focal spot, and a beam supply device, which supplies the laser radiation generated by the laser radiation source to the processing head. The beam supply device may comprise optical fibres or other optical waveguides and/or one or more deflecting mirrors with plane or curved surfaces. The processing head can be fastened to a movable robot arm or another moving device which enables a three-dimensional positioning. The laser radiation source is often arranged further away from the processing head or a moving device carrying the latter.
Hitherto the workpieces to be processed have usually been brought into a defined position by handling devices. The processing head is then guided with the aid of the robot at a distance of a few centimeters above the stationary workpiece. Since the processing head is heavy, it is not possible to perform very fast movements, as would be appropriate for example in spot welding operations. In principle, the workpiece could additionally be moved, but this increases the constructional expenditure on the handling devices.
In order to be able to process workpieces quickly at locations lying far apart, laser processing devices have therefore been developed in which the focal spot of the laser radiation is guided with the aid of a scanning device, which usually includes an arrangement of galvanomirrors, over the workpiece. If the processing head is far enough away (e.g. about 50 cm) from the workpiece, locations lying far apart on the workpiece can be processed extremely quickly by the laser radiation. The movement of the relatively heavy processing heads is thus replaced by movements of the light galvanomirrors in the scanning device. Processing methods in which the processing head is situated far away from the workpiece and include a scanning device are often called remote laser welding (or welding-on-the-fly) or remote laser cutting.
Besides the higher processing speed, these methods have the advantage that spatter and other contamination arising during the processing can hardly reach and contaminate the processing head any more. Protective glasses on the processing head thus need to be replaced less frequently, thereby reducing the downtimes. Moreover, the processing head no longer needs to be moved at all or if necessary moved only relatively slowly, making a robot redundant or more cost-effective.
A problem when using such laser processing devices which has hitherto not yet been satisfactorily solved is that it is difficult to position the focal spot, the diameter of which in welding processing mostly lies between 100 μm and 500 μm and in cutting processing may be 20 μm and less, precisely on the surface of the workpieces to be processed. Therefore, it has hitherto not been possible, for example, to produce fillet weld seams at lap joints of galvanised steel plates, because the focal spot cannot be positioned accurately enough in the fillet weld. For this reason, hitherto galvanised steel plates have mostly been joined by a laser deep welding process, in which an air gap must be left between the surfaces. This air gap is necessary so that the zinc coating melted on in an explosive manner can spread. The formation of cavities and defects along the weld seam can thereby be prevented. In order to keep the steel plates at a distance, they must have distance-maintaining indentations. The difficulty in reliably producing fillet weld seams thus ultimately leads to restrictions in the design of the workpieces and additional material consumption.
The reasons why the focal spot cannot be positioned accurately enough on the workpieces to be processed are as follows. Hitherto in remote laser processing the focal spot has been guided over the workpieces to be processed in accordance with a predetermined control program. Shape deviations of the workpieces themselves and positioning tolerances of the handling devices and optionally used robot, however, result in the location to be processed on the workpiece often not being situated at its desired position. Since such deviations are not taken into account in the control, the processing actually takes place outside the desired position.
It would be ideal if the focal spot could track the actually encountered spatial arrangement of the workpieces in a regulating process. For this purpose, however, it would be necessary to measure this actual spatial arrangement of the workpieces to be processed relative to the processing head or another reference point during the laser processing in real time. However, it has not been possible hitherto to carry out such a measurement successfully.
An observation of the processing location with the aid of a camera does not therefore lead to the desired improvements, because the camera captures only a 2D projection of the workpieces. If the beam path of the camera runs coaxially with the laser radiation, as is known in the prior art, although a lateral offset along the directions X and Y can be measured accurately, the distance between the workpiece and the processing head along the Z direction cannot be measured accurately. Because for high process quality, measuring accuracies are required in the Z direction which are in the order of around 400 μm for welding processing and in the order of around 100 μm for cutting processing.
For light-section or triangulation methods the distance between the processing head and the workpiece is too great to be able to measure with sufficient precision.
Chromatic-confocal measuring methods are also unsuitable, because on the one hand the numerical aperture of the focusing optics in the processing head is too low and on the other hand the chromatic longitudinal aberration thereof is too small to be able to cover a sufficient measuring range.
For distance measurement during the laser processing, some time ago the use of optical coherence tomographs (OCT) was proposed, cf. in particular EP 1 977 850 B1, DE 10 2010 016 862 B3 and DE 10 2012 207 835 A1. Optical coherence tomography enables high-precision distance measurement and even the generation of a 3D profile of the scanned surfaces when the measuring beam is guided scanner-like over the surfaces.
For remote laser processing in which the distance between the focal spot and the processing head can vary in the Z direction by up to 50 cm within fractions of a second, the optical coherence tomographs known in the prior art are, however, not suitable. Coherence tomographs which operate in the time domain (TD-OCT) usually contain a mirror in the reference arm of the coherence tomograph which modulates the optical path length thereof. The mirror vibrates at high frequency in the axial direction, whereby depth information can be obtained sequentially. The moving distance covered by the movable mirror is, however, only in the order of a few millimeters. The measuring range of such TD-OCTs is thus likewise only a few millimeters and would thus be a good two orders of magnitude too small for remote laser processing.
Coherence tomography in the frequency domain (FD-OCT), in which the optical path length in the reference arm is not changed, can also achieve a measuring range of only a few centimeters. For conventional laser processing devices in which the processing head is guided at an approximately constant distance over the workpieces, this measuring range is perfectly adequate. For remote laser processing, however, this measuring range is also insufficient.
From US 2012/0138586 A1 there is known a laser processing device having an OCT, in which the optical path length in a reference arm of the OCT can be tracked when the focal spot of the measuring beam is laterally deflected.
In DE 102 02 036 A1 there is described a laser processing device having a deflecting device and adjustable focusing optics. The focal spot of the laser radiation can thereby be moved in a manner which is plane and perpendicular to the beam direction.